Step 1: Understanding.
Slow thinkers struggle to adapt to the demands of practical life and hence are described as handicapped in earning a living.
Step 2: Checking options.
(A) Slow thinkers: Correct.
(B) High thinkers: Incorrect, they are idealists but not "handicapped."
(C) Fast thinkers: Incorrect.
(D) Middle-paced thinkers: Incorrect.
Step 3: Conclusion.
Answer is (A) Slow thinkers.